Circuit breakers provide overcurrent and short circuit protection for electric power systems. In the small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. Such a device includes a bimetal which is heated and bends in response to a persistent overcurrent condition thereby unlatching a spring powered operating mechanism which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature attracted by the sizable magnetic forces generated by a short circuit also unlatches, or trips, the operating mechanism.
In many applications, the miniature circuit breaker also provides ground fault protection. An electronic circuit detects leakage of current to ground and generates a ground fault trip signal. This signal energizes a shunt trip solenoid which unlatches the operating mechanism, typically through actuation of the thermal-magnetic trip device.
Recently, there has been considerable interest in also providing protection against arcing faults. Arcing faults are intermittent high impedance faults which can be caused for instance by worn insulation, loose connections, broken conductors, and the like. Because of their intermittent and high impedance nature, arcing faults do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnetic trip device. Consequently, separate electrical circuits have been developed for responding to arcing faults.
Ground fault protection circuits and arc fault protection circuits typically include test circuits for affirming their continued operability. These tests must be performed independently to assure operation of both functions. The simplest approach is to provide separate test switches, each with its own test button, for performing the ground fault and arc fault tests. However, the molded cases of the miniature circuit breakers have been standardized for interchangeable use in load centers. There is limited space available in the standardized miniature circuit breakers for all of the additional circuitry required for ground fault and arc fault protection, let alone the test circuits.
Commonly owned U.S. patent application Ser. No. 069,355 filed on Apr. 29, 1998 discloses an arrangement in which a common test button selectively actuates either the ground fault test circuit or the arc fault test circuit. The arc fault test switch and ground fault test switch comprise cantilevered electrically conductive flat springs straddling but spaced from a common flat spring. The free ends of all three of these flat springs are in parallel planes. The common test button is a rocker button rotatable in one direction from a neutral position to deflect the flat spring of the arc fault test switch into contact with the common flat spring to actuate the arc fault test circuit. When the rocker button is rotated in the other direction from the neutral position, the flat spring of the ground fault test switch is rotated into engagement with the common flat spring. One or more leaf springs secured in slots in the rocker button bias it to the neutral position. While this common rocker button separately actuates the two test circuits, its reliability is dependent upon tight manufacturing tolerances. Also, the biasing springs add additional labor and cost to manufacture of the circuit breaker.
There is a need, therefore, for an improved arrangement for selectively actuating an arc fault test circuit and a ground fault test circuit in a circuit breaker, and especially in a miniature circuit breaker.
There is a further need for such a circuit breaker which is easy to use, is inexpensive to manufacture, and does not require the holding of tight manufacturing tolerances.